This invention relates to an improved design of information recording media, for example in the form of light-readable discs, and an improved design of optical reading equipment used for reading such information recording media. These improved designs, among other advantages, will allow greater tolerances in manufacturing of the information recording media and will permit the use of less expensive optical reading equipment.
Information recording media in the form of light-readable discs are well known, as shown, for example in Kramer U.S. Pat. No. 5,068,846 and Mecca U.S. Pat. No. 5,995,481. Commercially available audio compact discs (xe2x80x9cCDsxe2x80x9d) and compact disc read-only memories (xe2x80x9cCD-ROMsxe2x80x9d) are examples of recording media of this general type.
The predominant portion of a typical light-readable disc comprises a transparent material, such as polycarbonate plastic. Information in the form of binary data is contained in a pit and land structure impressed along the top surface of this transparent material. The structure is covered by a very thin metal reflective layer, typically aluminum. A protective layer, typically lacquer, is then placed over the reflective layer, so that the protective layer fills the indentations in the reflective layer and provides a smooth, substantially planar upper surface for the disc on which a label or other information may be placed.
The pits and lands optical structure of the light-readable disc""s medium can be read by a laser beam focused on the reflective layer. The laser beam passes through the bottom of the transparent material and through the optical structure of pits and lands (which is seen by the laser from below as a series of bumps and lands), and is reflected off the reflective layer, through the transparent material and out of the medium to an optical reading structure.
Several publications conclude, based on elementary properties of interference between the incident and reflected waves, that the maximum extinction of the returned light is obtained when the light reflected by a pit is in antiphase with the light reflected by the surrounding land, namely, when the pit depth/bump height (depending on the plane of reference) is a quarter-wavelength. (G. Bouwhuis et al., Principles of Optical Disc Systems, 1st ed.(1985); K. Pohlmann, The Compact Disc, Updated ed. (1992); Dil et al., J. Opt. Soc. Am., 69:950 (1979)). This has led to the standardization of pit depth/bump height (depending upon the plane of reference) in commercial light-readable information recording media at just less than approximately one-quarter of the wavelength of the laser light within the transparent material.
However, recent findings revealed that the quarter wavelength criterion did not predict optimum results under all circumstances. As reported in Mecca U.S. Pat. No. 5,995,481, it had been determined that an improved light-readable recording disc was achieved by designing the pit depth/bump height to be approximately one-half of the wavelength of the laser light source. Not only such a disc had been found to be operable, but it had been determined that the intensity difference between the bump and land areas detected by an optical reader was actually greater than for the one-quarter wavelength pit previously thought to be optimal.
Numerous efforts had been undertaken to understand and/or explain the unexpected and improved results obtained by controlling pit height to approximately one-half the wavelength of light. None have borne fruit until now.
The present invention provides a way to optimize performance of both information recording media and the optical reading equipment by using a two-point-source model to simulate the waves reflected from the data surface of the recording media. The improved results promise substantial savings in the design and manufacture of less sensitive and less costly light-reading systems, as well as in disc manufacturing. Since a higher intensity signal difference is generated, manufacturing tolerances can be relaxed without sacrificing quality in the output and reading of data, leading to higher yields and lower costs.
In view of the foregoing, it is an object of this invention to provide a light-readable information recording medium and an optical reader that generate greater signal intensity difference, which can improve the quality of reproduction, reduce the cost and complication of optical reading devices, and provide for greater tolerance in the manufacturing of such recording media.
These and other objects of the present invention are accomplished as follows. According to the new model, two spherical (not plane) waves return from the data surface and travel to the objective lens. One spherical wave is due to the reflection of the read-out beam from the pit, the other is due to the reflection from the surrounding land. The returning field then results from the superposition of two spherical waves, which first diverge from the disc and are then focused by the objective lens onto the photodetector. Consequently, according to the model presented in this invention, one is dealing with interference of two converging spherical waves with slightly different foci, slightly different focal lengths and slightly different cone angles, rather than with interference of two plane waves. The focal regions of the two converging spherical waves are overlapping, and the interference of the two focused spherical waves takes place in the common region.
This two-point-source model was next applied to investigate how binary data recorded on a surface of an information recording medium are transferred into series of light pulses. The analysis resulted in an expression for the optical pit depth in terms of the system parameters for which destructive interference leads to a maximum extinction of the light in the focal region:             Δ      opt        =                  λ        2            ⁢              xe2x80x83            ⁢              m                  1          +                      M            T            2                                ,
wherein xcex is the wavelength of the light used to read the information recording medium, m is the order of interference selected from a group consisting of odd integers, and MT is the transverse magnification of the reader.
Accordingly, the present invention provides an information recording medium, which can be read by an optical reader using reflected light, comprising an optical data storage structure including pits and lands, in which the depth of the pits is equal to about:             λ              2        ⁢        n              ⁢          xe2x80x83        ⁢          m              (                  1          +                      M            T            2                          )              ,
wherein xcex is the wavelength of the light used to read the information recording medium, m is the order of interference selected from a group consisting of odd integers, n is the refractive index of the media encountered by the reading light inside the pits, and MT is the transverse magnification of the optical reader.
In addition, the invention facilitates building a system for optically reading stored information, which comprises an information recording medium having a light-reflecting surface formed as a data storage structure including pits and lands, a light source directed at the data storage structure so that the light is reflected in accordance with the information recorded using pits and lands, and an optical reader for detection of the reflected light and reading of the information recorded by means of pits and lands. According to the principles of the invention, the wavelength xcex of the light used to read the information recording medium, the interference order m selected from a group consisting of odd integers, the transverse magnification MT of the optical reader, the refractive index n of the media within the pits, and the depth of the pits d satisfy the following relationship:   nd  ≃            λ      2        ⁢          xe2x80x83        ⁢                  m                  1          +                      M            T            2                              .      
Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the invention.